As was predicted, CEO of McGraw-Hill has confirmed in an interview to CNBC today that Apple is going to announce their Tablet on 27th and it will run on iPhone OS. Terry McGraw, who is the CEO of the famous publishing company further went on to confirm that they “have worked with Apple for quite a while” and that it will be based on the “iPhone operating system”.

An anonymous iPhone app developer has dropped a hint today to CNET, stating that he has submitted an updated version of his app for iPhone OS 4.0 to Apple App Store for approval. This hint surely indicated towards a possibility where Apple might have already sent out the said firmware to a selected pool of testers.

Last week we informed you about the first evidence, which hinted towards the possible existence of iPhone OS 3.1.3, and iPhone 4.0 firmware. Today, MacRumors has been tipped by a developer who has spotted a single instance of iPhone OS 4.0 being used in his application’s Pinch Media analytics data. This discovery corresponds to the discovery of “iPhone 3,1” (the next-gen iPhone 4G model), which was spotted by the developer of iBart app last month.

The folks over at BGR have found out references to iPhone 3.1.3, and the iPhone 4.0 firmware version in their server logs and they seems to be legitimate. While iPhone 3.1.3 will only be a minor release, the 4.0 version certainly denotes a major release version and we can expect it to ship with the next-generation iPhone model sometime in summer 2010. Nothing much is known about about the features that the new firmware is going to offer, but we can expect it to add alot of new features, similar to what was experienced when firmware 3.0 was released.